In the preparation of certain semiconducting devices, it is common practice to grow a thin single crystalline film on a suitable crystalline substrate. The substrates and the films are selected so as to have nearly the same lattice spacing, i.e., nearly the same distance between atoms. Thus, the atom periodicity in the crystalline substrate acts as a template to define where the atoms for the crystalline film will deposit. If the films grow epitaxially (in registry), a single crystal film will result. Further, this film can be built upon sequentially to create the desired thickness of crystalline film material. In this art it is typical, for example, to grow single crystals of silicon and single crystals of germanium upon, respectively, substrates of silicon and gallium arsenide.
Several processes have been utilized in the prior art for the creation of these epitaxial crystalline films. One commonly used process is chemical vapor deposition (CVD). Generally, this involves introducing a gas containing the materials for deposit into an environment containing the substrate, which is heated, in order to decompose the gas molecules and release the atoms used to grow the films. Although this involves rather simple apparatus, the CVD film growth at low substrate temperature often results in a low deposition rate and in a nonuniform (rough) deposit. Higher substrate temperatures can be used to increase the CVD deposition rate but, in this case, there is often interdiffusion between the substrate and the deposit, resulting in various impurities being found in the deposit. Recently some improvement (increased deposition rate) in the CVD process has been made by utilizing plasma-assisted CVD deposition.
Another known method is referred to as "molecular beam epitaxy" (MBE). According to this method, atomic or molecular beams are generated by heating the solid elements in ovens (within the deposition chamber) from which a beam is emitted. In general, a separate oven is needed for each element that is to be deposited. Since the ovens are operated at elevated temperatures, the only control of vapor flow is through the use of mechanical shutters. Consequently, the elevated temperature portions of the apparatus must be elaborately shielded from the rest of the deposition chamber to minimize contamination of the deposit during slow growth. Because the atoms or molecules in these beams have only thermal velocities and the beams are of low intensity, MBE achieves a relatively slow deposition rate. A further disadvantage results from the necessity of opening the deposition chamber (and thus requiring re-evacuation) when the material in the oven is replaced. This method is described, for example, in "III-V MBE Growth Systems" by G. J. Davies and D. Williams, in The Technology and Physics of Molecular Beam Epitaxy, pp. 15-46, Plenum Press, NY (1985).
Still another process is "metalorganic molecular beam epitaxy" (MO-MBE). Gaseous sources (typically metalorganic molecules for elements in Column II and III, and hydrides for Column V and VI elements) are used. The sources are external to the deposition chamber, thus eliminating re-opening the deposition chamber to change sources. While gas flows can be switched more rapidly, mechanical shutters still are used to overcome switching transients common to mass flow controllers and to produce the desired very sharp on-off control that makes possible monolayer-by-monolayer growth. As with the MBE method, the deposition rate for the MO-MBE method is relatively slow because of the low velocity and intensity of the incident molecular beam. This method is typically described in "Metalorganic Molecular Beam Epitaxy (MOMBE)" by H. Luth in Institute of Physics Conference Series No. 82, ESSDERC 1986, Cambridge England, Sept. 8-11, 1986.
A further prior art process is referred to as "chemical beam epitaxy" (CBE). In CBE metalorganic molecules are used to supply all of the Column II, III, V and VI elements. These are transported to the deposition chamber from an exterior liquid source "bubbler" using hydrogen or another carrier gas. Mass flow controllers and mechanical shutters are used for flow control and gas switching as in MO-MBE. However, the deposition rate is relatively slow. This method is typically described in "Chemical Beam Epitaxy of InP and GaAs" by W. T. Tsang in Applied Physics Letters, 45, pp. 1234-1236 (1984).
Another reference that is deemed to be material to this invention is "New Approach to the Atomic Layer Epitaxy of GaAs Using a Fast Gas Stream" by M. Ozeki, K. Mochizuki, N. Ohtsuka and K. Kodama in Applied Physics Letters 53, pp. 1509-1511 (1988). This describes the use of a jet nozzle to produce fast gas flow to prevent decomposition of the molecules of gas in the stagnant boundary layer that forms just above the film surface in a normal slow-moving gas stream. While switching of the jet flow is described, to limit film growth to one atomic layer at a time, the switching time is of the order of 1 to 10 seconds. Very slow deposition rates are reported for this method.
Accordingly, it is an object of the present invention to provide a method (and apparatus) for achieving controlled deposition of films upon a substrate with high deposition rates.
It is another object of the present invention to provide for a controlled growth mode for producing epitaxial single crystals upon a crystalline substrate.
A further object is to provide a method for producing films having improved thickness uniformity and reduced roughness.
Also, it is an object to provide a method for achieving fabrication of sharp interfaces between thin film layers. Yet a further object is to provide a method to use a directional beam of molecules to confine deposition to the region of the heated substrate.
Another object is to provide a method to enhance deposition on a heated substrate while minimizing deposition on the walls of the deposition chamber.
An additional object of the present invention is to provide for the reduced cost of the deposition of films for producing semiconductor and similar electronic devices.
These and other objects of the invention will become apparent upon a consideration of the figures referred to hereinafter together with a complete description thereof.